An Innovative Hydrologic and Hydraulic Modeling Approach for MWRDGC's Calumet Tunnel and Reservoir Plan (TARP) System

In 1972, the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) adopted the Tunnel and Reservoir Plan (TARP), as the Chicago area's long-term control plan to cost-effectively comply with Federal and State water quality standards in the 375 square-mile combined sewer area consisting of Chicago and 51 suburbs. TARP's main goals are to protect Lake Michigan – the region's drinking water supply - from raw sewage pollution; improve water quality of area rivers and streams; and provide an outlet for floodwaters to reduce street and basement sewage backup flooding.The TARP system consists of 109.4 miles of deep rock bored, concrete-lined tunnels that range in diameter from 8 feet to 33 feet and are between 150 ft and 350 ft below ground level. A combination of stormwater and wastewater enters the system through 259 drop shafts (between 4 ft and 25 ft in diameter), passing through over 600 near-surface connecting and regulating structures placed throughout the system. There are four main TARP systems, Upper Des Plaines (O'Hare), Des Plaines, Mainstream and Calumet. The system also includes three major pumping stations and three large open-air reservoirs. As designed, the total combined sewer overflow (CSO) storage capacity of the tunnels and reservoirs is 17.5 billion gallons. Construction of the TARP system began in 1975 and continues today with the Thornton Reservoir due for completion in 2015 and Stage 1 and 2 of the McCook reservoir due in 2017 and 2029, respectively.Since the design of the TARP, no comprehensive hydrologic/hydraulic study of the system has been undertaken. In 2003, MWRDGC approached the University of Illinois at Urbana Champaign (UIUC) to develop a new, updated and enhanced computer model for each TARP system, to allow real-time evaluation of the TARP systems. The new real-time flow model will optimize operation of the system as actually constructed, determine constraints in the system, identify physical changes needed to improve performance, and allow “what-if” analyses to be performed for potential storm scenarios and facility revisions.Although simple in concept, modeling the TARP system poses challenges that cannot be resolved using any single existing modeling tool. These challenges include: the size and complexity of the TARP system, interceptor system and its contributing service area; lack of detailed hydrologic and hydraulic input data; and the propensity of the system to generate hydraulic conditions that can cause geysering to occur. UIUC has helped MWRDGC develop a set of modeling tools, which combine to form a simulation package capable of simulating the wide-range of hydrologic and hydraulic conditions that the system can be subjected to. This tool has been used to evaluate the Calumet TARP system and is in the process of being applied to the Mainstream/Des Plaines TARP system. It has allowed the District to identify how the system will behave once the reservoirs come online and provides them with a tool to conduct “what-if” scenario analysis that will allow them to optimize operation of the system. Through investment in the development of technology, MWRDGC will have tools that can be used to optimize the TARP system, thereby reducing flooding and combined sewer overflows, and improving the water quality of surrounding waterways. This paper presents the simulation package that has been developed and its application to the Calumet TARP system.